1. Field of the Invention
The present invention relates to a semiconductor device using a gallium nitride-based compound semiconductor material and, more particularly, to a gallium nitride-based compound semiconductor laser for emitting short-wavelength light, and a method of manufacturing the same.
2. Discussion of the Background
In recent years, gallium nitride-based compound semiconductors such as GaN have received a great deal of attention as a material for a short-wavelength light-emitting diode and a semiconductor laser from the blue to ultraviolet range. Particularly, a blue semiconductor laser using this material series has been expected to be applied as a light source for high-density information processing because of its short oscillation wavelength.
To oscillate a semiconductor laser with a low-threshold current, carrier injection into an active layer, and light confinement must be efficiently performed. To efficiently inject carriers into the active layer, a double-heterojunction having a p-n junction, and a current constriction structure are important. To efficiently confine light, formation of an optical waveguide having a large refractive index difference is important.
In a gallium nitride-based compound semiconductor laser, however, only formation of an optical waveguide by a mesa, or a surface emission semiconductor laser has been proposed. Any other structures and manufacturing methods have rarely been reported. In this conventional semiconductor laser, carrier injection and light confinement are not sufficient. In addition, in a mesa optical waveguide, the p-side contact area is small, resulting in high-device-resistance characteristics. With another compound semiconductor material, carrier injection and light confinement can be efficiently performed by filling the two sides of the mesa with current blocking layers. With the gallium nitride-based compound semiconductor material, such an arrangement cannot be directly applied.
This is because the gallium nitride-based compound semiconductor material is difficult to cause crystal growth and to obtain a high crystal quality. For this reason, the mesa is difficult to be formed into a desired shape by exposing a desired surface by etching. Further, the gallium nitride-based compound semiconductor material is very difficult to be regrown at a step portion formed upon mesa etching. As described above, the mesa is difficult to be formed into a desired shape by exposing a desired surface by mesa etching. As a result, the two sides of the mesa becomes more difficult to be buried.
In this manner, in the semiconductor laser using the gallium nitride-based compound semiconductor material, carrier injection and light confinement are not always sufficient, and the threshold cannot be decreased. In the mesa optical waveguide, the contact area of a contact portion (generally on the p side) opposite to the substrate is small, resulting in high-device-resistance characteristics.